CCD camera with a plurality of sensor areas and corresponding output channels correcting image using a clamping process

ABSTRACT

An electronic camera includes a CCD imager. The CCD imager has an imaging surface in which two sensor areas are formed. A raw image produced in one sensor area is output from a channel CH 1 , and a raw image produced in the other sensor area is output from a channel CH 2 . The raw image from the channel CH 1  is clamped by a clamp circuit at a clamp level LV 1 , and the raw image from the channel CH 2  is clamped by another clamp circuit at a clamp level LV 2 . The clamp level LV 1  matches an optical black level VCL of the raw image from the channel CH 1 . The clamp level LV 2  matches a subtraction level “VCL−(VAL−VAR)” determined by subtracting a difference between a preliminary feeding level VAL of the raw image from the channel CH 1  and a preliminary level VAR of the raw image from the channel CH 2 , from the black level VCL of the raw image from the channel CH 1.

CROSS REFERENCE OF RELATED APPLICATION

The disclosure of Japanese Patent Application No. 2005-030256 isincorporated herein by reference.

BACKGROUND OF THE INTENTION

1. Field of the Invention

The present invention relates to an electronic camera. Morespecifically, the present invention relates to an electronic camera thatsubjects an image signal output from an image sensor to a clamp process.

2. Description of the Prior Art

One example of this kind of conventional apparatus is disclosed inpatent document 1 (Japanese Patent Laid-open No. 2003-259224). Inaccordance with this conventional art, two adjacent imaging areas areformed on an imaging surface, and a surveillance area is assigned tothose two imaging areas in such a manner as to straddle a boundary linebetween them. A difference in level between two image signals generatedat the two imaging areas is determined with attention given to thesurveillance area to determine. Each of the two image signals isprovided with some gain so as to dissolve the level difference. Thismakes less visible a difference of grade in a synthetic image based onthe two image signals.

However, the two image signals derived from the imaging elements areoutput from different paths, which results in a discrepancy betweenlevels of noises superimposed on the image signals in their paths. Inthe conventional art, no consideration is given to such a discrepancy innoise level and thus there is a limit to obtaining an improvement inquality of synthetic image.

SUMMARY OF THE INVENTION

Therefore, it is a primary object of the present invention to provide anovel electronic camera.

It is another object of the present invention to provide an electroniccamera that is capable of further improving image quality.

According to claim 1, an electronic camera comprises: an imager havingan imaging surface in which a first photoelectric conversion area and asecond photoelectric conversion area are formed and a first output pathand a second output path assigned to the first photoelectric conversionarea and the second photoelectric conversion area, respectively; a firstdriver for outputting from the first output path a first partial imageproduced in the first photoelectric conversion area; a second driver foroutputting from the second output path a second partial image producedin the second photoelectric conversion area; a first damper forsubjecting the first partial image output from the first output path toa clamp process according to a first clamp level; a second damper forsubjecting the second partial image output from the second output pathto a clamp process according to a second clamp level; a first setter forsetting a black level of the first partial image output from the firstoutput path as a first clamp level; a subtracter for subtracting adifference between a preliminary feeding level of the first partialimage output from the first output path and a preliminary feeding levelof the second partial image output from the second output path, from theblack level of the first partial image output from the first outputpath; and a second setter for setting the level determined by thesubtracter as the second clamp level.

The imager has the imaging surface in which the first photoelectricconversion area and the second photoelectric conversion area are formedand the first output path and the second output path assigned to thefirst photoelectric conversion area and the second photoelectricconversion area, respectively. The first partial image produced in thefirst photoelectric conversion area is output by the first driver fromthe first output path, and the second partial image produced in thesecond photoelectric conversion area is output by the second driver fromthe second output path.

The first partial image output from the first output path is subjectedby the first damper to the clamp process according to the first clamplevel, and the second partial image output from the second output pathis subjected by the second damper to the clamp process according to thesecond clamp level. The first clamp level is set by the first setter,and the second clamp level is set by the second setter.

The first clamp level matches the black level of the first partial imageoutput from the first output path. On the other hand, the second clamplevel matches the subtraction level determined by subtracting adifference between the preliminary feeding level of the first partialimage output from the first output path and the preliminary feedinglevel of the second partial image output from the second output path,from the black level of the first partial image output from the firstoutput path. Besides, the subtraction level is calculated by thesubtracter.

The first partial image and the second partial image are output from thedifferent output path, and thus which may cause a discrepancy betweenthe levels of noises superimposed on the first partial image and thesecond partial image output from the imaging means, that is, the levelsof external noises.

Here, the level of the external noise superimposed on the first partialimage can be equated with the preliminary feeding level of the firstpartial image. The level of the external noise superimposed on thesecond partial image can be equated with the preliminary feeding levelof the second partial image. Accordingly, the difference between thepreliminary feeding level of the first partial image and the preliminaryfeeding level of the second partial image can be regarded as adiscrepancy amount of external noise.

Thus, in the present invention of claim 1, the black level of the firstpartial image is taken as the first clamp level, while the leveldetermined by subtracting the difference between the preliminary levelsfrom the black level of the first partial image is assumed as the secondclamp level. This dissolves the discrepancy between the clamp levelsresulting from the external noises.

In addition, both the first clamp level and the second clamp level areset according to the black level of the first partial image. Thiseliminates the discrepancy between the clamp levels resulting from thedifference between the black level of the first partial image and theblack level of the second partial image.

According to the electronic camera of claim 2 depending on to claim 1,the first driver and the second driver output the first partial imageand the second partial image, respectively, in a manner of rasterscanning. This generates periodically the preliminary feeding timeperiod, thereby making it easy to detect the preliminary feeding level.

According to the electronic camera of claim 3 depending on claim 1, thefirst photoelectric conversion area has an optical black area, and thefirst setter includes the black level detector for detecting the blacklevel with timing corresponding to the optical black area. This makes itpossible to adjust accurately the first clamp level and the second clamplevel.

The electronic camera of claim 4 depending on claim 1 further comprisesthe producer for producing a screenful of still image based on the firstpartial image subjected by the first damper to a clamp process and thesecond partial image subjected by the second damper to a clamp process.

According to the electronic camera of claim 5 depending on claim 4, thefirst driver and the second driver output periodically the first partialimage and the second partial image, respectively, and the producerproduces periodically the still image. This makes the output of movingimages.

According to the electronic camera of claim 6 depending on claim 1, thefirst photoelectric conversion area and the second photoelectricconversion area are adjacent to each other on the imaging surface.

According to the electronic camera of claim 7 depending on claim 1, thesubtracter includes the black level detector for detecting the blacklevel of the first partial image output from the first output path, thefirst preliminary feeding level detector for detecting the preliminaryfeeding level of the first partial image output from the first outputpath, the second preliminary feeding level detector for detecting thepreliminary feeding level of the second partial image output from thesecond output path, the level subtracter for subtracting the preliminaryfeeding level detected by the first preliminary feeding level detectorfrom the black level detected by the black level detector, and the leveladder for adding result of subtraction by the first level subtracter tothe preliminary feeding level detected by the second preliminary feedinglevel detector.

The above described objects and other objects, features, aspects andadvantages of the present invention will become more apparent from thefollowing detailed description of the present invention when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing configuration of one embodiment of thepresent invention;

FIG. 2 is an illustrative view showing one example of configuration ofan image sensor applied to the FIG. 1 embodiment;

FIG. 3(A) is a timing chart showing one example of output from a channelCH1 of the image sensor;

FIG. 3(B) is a timing chart showing one example of output from a channelCH2 of the image sensor;

FIG. 4 is an illustrative view showing one part of operation of the FIG.1 embodiment;

FIG. 5 is a graph showing one example of changes in signal level withrespect to light amount; and

FIG. 6 is a block diagram showing one example of configuration of aclamp level adjustment circuit applied to the FIG. 1 embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a digital camera (electronic camera) of thisembodiment includes an optical lens 12. An optical image of object sceneis irradiated to an imaging surface of a CCD imager 14 via the opticallens 12, and is subjected to photoelectric conversion by means of aplurality of light-receiving elements formed on the imaging surface. Onelight-receiving element is equivalent to one pixel.

The imaging surface is covered with a Bayer pattern of color filter (notshown). Accordingly, electric charges produced by the light-receivingelements to which a filter element of R (Red) is assigned have colorinformation of R, electric charges produced by the light-receivingelements to which a filter element of G (Green) is assigned have colorinformation of G, and electric charges produced by the light-receivingelements to which a filter element of B (Blue) is assigned have colorinformation of B.

A TG (Timing Generator) 18 generates a plurality of timing signalsincluding a horizontal synchronizing signal Hsync and a verticalsynchronizing signal Vsync. The drivers 16 a and 16 b each drive the CCDimager 14 in response to those timing signals. The electric chargesgenerated on the imaging surface, that is, the raw image signals areoutput from the CCD imager 14 at the rate of one frame per 1/30 second.

Referring to FIG. 2, the imaging surface of the CCD imager 14 has asensor area (photoelectric conversion area) in which the plurality oflight-receiving elements are arranged, a left non-sensor area and aright non-sensor area in which no light-receiving elements are arranged.An imaging area is formed by an area where several pixels at both endsin the horizontal direction are excluded from the sensor area. A left OB(Optical Black) area is formed by several pixels at left end of thesensor area and the left non-sensor area. The right OB area is formed byseveral pixels at right end of the sensor area and the right non-sensorarea. In addition, the left OB area and the right OB area are the samein size.

The sensor area has a left sensor area and a right sensor area. The leftsensor area is formed on the left of a boundary line BL extending fromcenter of the imaging surface in the vertical direction, and the rightsensor area is formed on the right of the same boundary line BL.Accordingly, the left sensor area and the right sensor area are adjacentto each other on the boundary line BL. A plurality of vertical transferregisters, although not shown, are assigned to the left sensor area andthe right sensor area. Besides, vertical transfer registers are alsoassigned to the left non-sensor area and the right non-sensor area.

Horizontal transfer registers HL are connected to ends of the verticaltransfer registers arranged on the left of the boundary line BL. Also,horizontal transfer registers HR are connected to ends of the verticaltransfer registers arranged on the right of the boundary line BL. Thus,the electric charges generated by the plurality of light-receivingelements on the left sensor area are transferred by the verticaltransfer registers not illustrated and the horizontal transfer registersHL, and then are output through an amplifier 14 a. Likewise, theelectric charges generated by the plurality of light-receiving elementson the right sensor area are transferred by the vertical transferregisters not illustrated and the horizontal transfer registers HR, andthen are output through an amplifier 14 b.

More specifically, the driver 16 a subjects the left sensor area toraster scanning based on the timing signals from the TG 18, and outputsa raw image signal for left ½ frame through a channel CH1. In similarfashion, the driver 16 b subjects the right sensor area to rasterscanning based on the timing signals from the TG 18, and outputs a rawimage signal for right ½ frame through a channel CH2.

However, a direction of transfer by the horizontal transfer registers HRis the reverse of a direction of transfer by the horizontal transferregisters HL. Therefore, a direction of raster scanning is invertedbetween the left sensor area and the right sensor area.

As a result, output of the channel CH1 changes in a manner shown in FIG.3(A), and output of the channel CH2 changes in a manner shown in FIG.3(B). According to FIG. 3(A), time periods AL, BL, CL and DL comerepeatedly in this order. According to FIG. 3(B), time periods AR, BR,CR and DR come repeatedly in this order. Among them, the time periods ALand AR are preliminary feeding time periods resulting from the rasterscanning, and the time periods BL and BR are time periods correspondingto the non-sensor areas forming the OB areas. In addition, the timeperiods CL and CR are time periods in correspondence with one part ofthe sensor area forming the OB areas, and the time periods DL and DR aretime periods corresponding to another part of the sensor area formingthe imaging area.

Therefore, the raw image signal from the channel CH1 shows a preliminaryfeeding level VAL at the period AL, an optical black level VBL at theperiod BL, an optical black level VCL at the period CL, and an effectiveimage level VDL at the period DL. Likewise, the raw image signal fromthe channel CH2 shows a preliminary feeding level VAR at the period AR,an optical black level VBR at the period BR, an optical black level VCRat the period CR, and an effective image level VDR at the period DR.

Referring to FIG. 4, the preliminary feeding level VAL includes noisecaused in the amplifier 14 a, and the preliminary feeding level VARincludes noise caused in the amplifier 14 b. The optical black level VBLfurther includes noise caused in the vertical transfer registers and thehorizontal transfer registers HL assigned to the left non-sensor area,and the optical black level VBR further includes noise caused in thevertical transfer registers and the horizontal transfer registers HRassigned to the left non-sensor area. The optical black level VCLfurther includes noise caused in the light-receiving elements, and theoptical black level VCR further includes noise caused in thelight-receiving elements.

The noise caused on the imaging surface of the CCD imager 14(light-receiving elements, vertical transfer registers) does not differsignificantly from place to place. Also, the noise caused in thehorizontal transfer registers HL and HR show no greatly largedifferences. However, the noise caused in the amplifiers 14 a and 14 bmay make great differences due to variations in amplificationcharacteristics. As a consequence, the preliminary feeding levels VALand VAR may differ from each other as shown in FIG. 4.

Referring to FIG. 5, a light amount-raw image level characteristic ofthe channel CH1 is defined by a straight line L1, and a light amount-rawimage level characteristic of the channel CH2 is defined by a straightline L2. A difference in gradient between the straight lines L1 and L2results from variations in amplification factor of the amplifiers 14 aand 14 b. Moreover, if the light amount is zero, the level differencebecomes “VAL−VAR”.

Returning to FIG. 1, a CDS/AGC/AD circuit 20 subjects the raw imagesignal from the channel CH1 to correlation double sampling, automaticgain adjustment and A/D conversion. In a similar way, a CDS/AGC/ADcircuit 28 subjects the raw image signal from the channel CH2 tocorrelation double sampling, automatic gain adjustment and A/Dconversion. Additionally, the CDS/AGC/AD circuits 20 and 28 execute theprocesses in synchronization with the timing signals from the TG 18.

An adder 22 forming a clamp circuit 42 inputs raw image data output fromthe CDS/AGC/AD circuit 20 and offset data held by a register 26, andsubtracts a data value of the offset data from a data value of the rawimage data. Likewise, an adder 30 forming a clamp circuit 44 inputs rawimage data output from the CDS/AGC/AD circuit 28 and offset data held bya register 34, and subtracts a data value of the offset data from a datavalue of the raw image data. The raw image data from the adders 22 and30 have subtracted data values. The raw image data output from the adder22 is given to a plus terminal of an arithmetic unit 24, and the rawimage data output from the adder 30 is provided to a plus terminal of anarithmetic unit 32.

A clamp level adjustment circuit 36 provides clamp level data (datavalue: LV1) to a minus terminal of the arithmetic unit 24 and providesclamp level data (data value: LV2) to a minus terminal of the arithmeticunit 32.

The arithmetic unit 24 subtracts the data value LV1 of the clamp leveldata from the data value of the raw image data from the channel CH1. Outof the subtracted data output from the arithmetic unit 24, the register26 holds the subtracted data belonging to the time period CL shown inFIG. 3(A) as the above mentioned offset data. The arithmetic unit 32also subtracts the data value LV2 of the clamp level data from the datavalue of the raw image data from the channel CH2. Out of the subtracteddata output from the arithmetic unit 32, the register 34 holds thesubtracted data belonging to the time period CR shown in FIG. 3(B) asthe above mentioned offset data.

Consequently, the raw image data from the channel CH1 is subjected to adigital clamp process according to the data value LV1, and the raw imagedata from the channel CH2 is subjected to a digital clamp processaccording to the data value LV2. That is, the black level of the rawimage from the channel CH1 is set at the clamp level LV1, and the blacklevel of the raw image from the channel CH2 is set at the clamp levelLV2.

A signal processing circuit 38 subjects the raw image data output fromthe adders 22 and 30 to a series of processes including channelmatching, color separation, white balance adjustment, YUV conversion andNTSC coding to produce a composite video signal in NTSC format. Theproduced composite video signal is provided to an LCD monitor 40. TheCCD imager 14 is driven at a frame rate of 30 fps, and thus images ofobject scene in smooth motion are output from the LCD monitor 40.

The clamp level adjustment circuit 36 is configured as shown in FIG. 6.The raw image signal from the channel CH1 output through the CCD imager14 is given to switches SW1 and SW2. The raw image signal from thechannel CH2 output through the CCD imager 14 is given to a switch SW3.

The TG 18 turns on the switches SW1 and SW2 at the time periods CL andAL shown in FIG. 3(A), respectively, and turns on the switch SW3 at thetime period AR shown in FIG. 3(B). Therefore, the optical black levelVCL of the raw image signal from the channel CH1 is held by a capacitorC1, the preliminary feeding level VAL of the raw image signal from thechannel CH1 is held by a capacitor C2, and the preliminary feeding levelVAR of the raw image signal from the channel CH2 is held by a capacitorC3.

The optical black level VCL held by the capacitor C1 is given via abuffer 36 a to the A/D converter 36 f and a plus terminal of thearithmetic unit 36 d. Also, the preliminary feeding level VAL held bythe capacitor C2 is provided via a buffer 36 b to a minus terminal ofthe arithmetic unit 36 d. Moreover, the preliminary feeding level VARheld by the capacitor C3 is provided via a buffer 36 c to a plusterminal of the arithmetic unit 36 e.

The arithmetic unit 36 d subtracts the preliminary feeding level VALfrom the optical black level VCL, and provides a subtraction level“VCL−VAL” to the other plus terminal of the arithmetic unit 36 e. Thearithmetic unit 36 e adds up the preliminary feeding level VAR and thesubtraction level “VCL−VAL”, and provides a subtraction level“VCL−(VAL−VAR)” to the A/D converter 36 g.

The A/D converter 36 f outputs clamp level data with a data valuecorresponding to the optical black level VCL (=LV1). In addition, theA/D converter 36 g outputs clamp level data with a data valuecorresponding to the subtraction level “VCL−(VAL−VAR)” (=LV2). Moreover,the digital clamp process according to those clamp level data is carriedout in the above described procedure. As a consequence, the lightamount-raw image level characteristic of the channel CH1 changes fromthe straight line L1 to L1′ shown in FIG. 5. The light amount-raw imagelevel characteristic of the channel CH2 changes from the straight lineL2 to L2′ shown in FIG. 5.

As can be understood from the above description, the CCD imager 14 hasthe imaging surface in which the left sensor area (the firstphotoelectric conversion area) are formed and the right sensor area (thesecond photoelectric conversion area) and the horizontal transferregisters HL and HR assigned to the left sensor area and the rightsensor area, respectively. The raw image (the first partial image) fromthe channel CH1 produced in the left sensor area is output through theamplifier 14 a when it is driven by the driver 16 a (the first driver),the raw image (the second partial image) from the channel CH2 producedin the right sensor area is output through the amplifier 14 b when it isdriven by the driver 16 b (the second driver).

The raw image from the channel CH1 is subjected by the clamp circuit 42(the first clamper) to a clamp process according to the clamp level LV1(the first clamp level). Also, the raw image from the channel CH2 issubjected by the clamp circuit 44 (the second clamper) to a clampprocess according to the clamp level LV2 (the second clamp level). Here,the clamp level LV1 is set by the A/D converter 36 f (the first setter)forming the clamp level adjustment circuit 36. In addition, the clamplevel LV2 is set by the A/D converter 36 g (the second setter) formingthe clamp level adjustment circuit 36.

The clamp level LV1 matches the optical black level VCL of the raw imagefrom the channel CH1. On the other hand, the clamp level LV2 matches thesubtraction level “VCL−(VAL−VAR)” found by subtracting a differencebetween the preliminary feeding level VAL of the raw image from thechannel CH1 and the preliminary feeding level VAR of the raw image fromthe channel CH2, from the black level VCL of the raw image from thechannel CH1. Besides, the subtraction level “VCL−(VAL−VAR)” iscalculated by the arithmetic units 36 d and 36 e (the subtracters).

The two raw images are output from the different channels CH1 and CH2,respectively, which may cause a discrepancy between levels of noisessuperimposed on the raw images, that is, levels of external noises.

Here, the level of the external noise superimposed on the raw image fromthe channel CH1 can be equated with the preliminary feeding level VAL ofthe raw image from the channel CH1. The level of the external noisesuperimposed on the raw image from the channel CH2 can be equated withthe preliminary feeding level VAR of the raw image from the channel CH2.Accordingly, the difference between the preliminary feeding levels VALand VAR can be regarded as a discrepancy amount of external noise.

Thus, in this embodiment, the optical black level VCL of the raw imagefrom the channel CH1 is taken as the clamp level LV1, while thesubtraction level “VCL−(VAL−VAR)” determined by subtracting thedifference between the preliminary levels VAL and VAR from the opticalblack level VCL of the raw image from the channel CH1 is assumed as theclamp level LV2. This dissolves the discrepancy between the clamp levelsLV1 and LV2 resulting from the external noises.

In addition, the both clamp levels LV 1 and LV2 are set according to theoptical black level VCL of the raw image from the channel CH1. Thiseliminates the discrepancy between the clamp levels LV1 and LV2resulting from the difference between the optical black level VCL of theraw image from the channel CH1 and the optical black level VCR of theraw image from the channel 2. As a result, it is possible to improve thequality of an image to be displayed on the LCD monitor 40.

Besides, the sensor area formed on the imaging surface is divided intotwo in this embodiment, and alternatively, the sensor area may bedivided into three or more. Moreover, the left OB area and the right OBarea are the same in size in this embodiment, and alternatively, theymay have different sizes. Furthermore, this embodiment employs digitalclamp operation, and the present invention may also apply to analogclamp operation.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. An electronic camera, comprising: an imager having an imaging surfacein which a first photoelectric conversion area and a secondphotoelectric conversion area are formed and a first output path and asecond output path assigned to said first photoelectric conversion areaand said second photoelectric conversion area, respectively; a firstdriver for outputting from said first output path a first partial imageproduced in said first photoelectric conversion area; a second driverfor outputting from said second output path a second partial imageproduced in said second photoelectric conversion area; a first damperfor subjecting the first partial image output from said first outputpath to a clamp process according to a first clamp level; a seconddamper for subjecting the second partial image output from said secondoutput path to a clamp process according to a second clamp level; afirst setter for setting a black level of the first partial image outputfrom said first output path as the first clamp level; a subtracter forsubtracting a difference between a preliminary feeding level of thefirst partial image output from said first output path and a preliminaryfeeding level of the second partial image output from said second outputpath, from the black level of the first partial image output from saidfirst output path; and a second setter for setting the level determinedby said subtracter as the second clamp level.
 2. An electronic cameraaccording to claim 1, wherein said first driver and said second driveroutput the first partial image and the second partial image,respectively, in a manner of raster scanning.
 3. An electronic cameraaccording to claim 1, wherein said first photoelectric conversion areahas an optical black area, and said first setter includes a black leveldetector for detecting the black level with timing corresponding to saidoptical black area.
 4. An electronic camera according to claim 1,further comprising a producer for producing one screen of still imagebased on the first partial image subjected by said first damper to aclamp process and the second partial image subjected by said seconddamper to a clamp process.
 5. An electronic camera according to claim 4,wherein said first driver and said second driver output periodically thefirst partial image and the second partial image, respectively, and saidproducer produces periodically the still image.
 6. An electronic cameraaccording to claim 1, wherein said first photoelectric conversion areaand said second photoelectric conversion area are adjacent to each otheron said imaging surface.
 7. An electronic camera according to claim 1,wherein said subtracter includes a black level detector for detectingthe black level of the first partial image output from said first outputpath, a first preliminary feeding level detector for detecting thepreliminary feeding level of the first partial image output from saidfirst output path, a second preliminary feeding level detector fordetecting the preliminary feeding level of the second partial imageoutput from said second output path, a level subtracter for subtractingthe preliminary feeding level detected by said first preliminary feedinglevel detector from the black level detected by said black leveldetector, and a level adder for adding result of subtraction by saidfirst level subtracter to the preliminary feeding level detected by saidsecond preliminary feeding level detector.